A major portion of the worldwide petrochemical industry is in involved with the production of light olefin materials and their subsequent use in the production of numerous important chemical products via polymerization, oligomerization, alkylation and the like well-known chemical reactions. Light olefins include ethylene, propylene and combinations thereof. These light olefins are essential building blocks for the modern petrochemical and chemical industries. The major source for these materials in present day refining is the stream cracking of petroleum feeds. For various reasons including geographical, economic, political and diminished supply considerations, the art has long sought a source other than petroleum for the massive quantities of raw materials that are needed to supply the demand for these light olefin materials.
The search for alternative materials for light olefin production has led to the use of oxygenates such as alcohols and, more particularly, to the use of methanol, ethanol, and higher alcohols or their derivatives such as dimethyl ether, diethyl ether, etc., for example. Molecular sieves such as microporous crystalline zeolite and non-zeolite catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures, particularly hydrocarbon mixtures composed largely of light olefins.
Such processing of oxygenates to form light olefins is commonly referred to as a methanol-to-olefin (MTO) process, as methanol alone or together with other oxygenate materials such as dimethyl ether (DME) is typically an oxygenate material most commonly employed therein. In practice, such oxygenate conversion processing arrangements commonly produce ethylene and propylene as main products and, as stand alone processing, can achieve propylene to ethylene product ratios up to about 1.4. In addition to the production of ethylene and propylene as main products, such processing also typically produces or results in smaller relative amounts of highly olefinic C4 and heavier hydrocarbon streams.
Commonly assigned U.S. Pat. No. 5,990,369 to Barger et al. discloses a process for the production of light olefins comprising olefins having from 2 to 4 carbon atoms per molecule from oxygenate feedstock. The process comprises passing the oxygenate feedstock to an oxygenate conversion zone containing a metal aluminophosphate catalyst to produce a light olefin stream. The light olefin stream is fractionated and a portion of the products are metathesized to enhance the yield of ethylene, propylene and/or butylene products. Propylene can be metathesized to produce an additional quantity of ethylene, or a combination of ethylene and butene can be metathesized to produce an additional quantity of propylene. The combination of light olefin production and metathesis or disproportionation is disclosed as providing flexibility such as to overcome the equilibrium limitations of the metal aluminophosphate catalyst in the oxygenate conversion zone. In addition, the invention thereof is disclosed as providing the advantage of extended catalyst life and greater catalyst stability in the oxygenate conversion zone.
While such processing can desirably result in the formation of increased relative amounts of propylene, further improvements such as to further enhance the relative amount of propylene production and recovery are desired and have been sought.